In today's radio communications networks a number of different technologies are used, such as Long Term Evolution (LTE), LTE-Advanced, Wideband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WiMax), or Ultra Mobile Broadband (UMB), just to mention a few possible technologies for radio communication. A radio communications network comprises radio base stations providing radio coverage over at least one respective geographical area forming a cell. The cell definition may also incorporate frequency bands used for transmissions, which means that two different cells may cover the same geographical area but using different frequency bands. User equipments (UE) are served in the cells by the respective radio base station and are communicating with respective radio base station. The user equipments transmit data over an air or radio interface to the radio base stations in uplink (UL) transmissions and the radio base stations transmit data over an air or radio interface to the user equipments in downlink (DL) transmissions.
Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to evolve the WCDMA standard towards the fourth generation (4G) of mobile telecommunication networks. In comparisons with third generation (3G) WCDMA, LTE provides increased capacity, much higher data peak rates and significantly improved latency numbers. For example, the LTE specifications support downlink data peak rates up to 300 Mbps, uplink data peak rates of up to 75 Mbit/s and radio access network round-trip times of less than 10 ms. In addition, LTE supports scalable carrier bandwidths from 20 MHz down to 1.4 MHz and supports both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) operation.
LTE is a Frequency Division Multiplexing technology wherein Orthogonal Frequency Division Multiplexing (OFDM) is used in a DL transmission from a radio base station to a user equipment. Single Carrier—Frequency Domain Multiple Access (SC-FDMA) is used in an UL transmission from the user equipment to the radio base station. Services in LTE are supported in the packet switched domain. The SC-FDMA used in the UL is also referred to as Discrete Fourier Transform Spread (DFTS)-OFDM.
The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each Resource Element (RE) corresponds to one OFDM subcarrier during one OFDM symbol interval. A symbol interval comprises a cyclic prefix (cp), which cp is a prefixing of a symbol with a repetition of the end of the symbol to act as a guard band between symbols and/or facilitate frequency domain processing. Frequencies f or subcarriers having a subcarrier spacing Δf are defined along an z-axis and symbols are defined along an x-axis.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame comprising ten equally-sized subframes, #0-#9, each with a Tsubframe=1 ms of length in time as shown in FIG. 2. Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with resource block 0 from one end of the system bandwidth.
Downlink and uplink transmissions are dynamically scheduled, i.e. in each subframe the radio base station transmits control information about to or from which user equipments data is transmitted and upon which resource blocks the data is transmitted. The control information for a given user equipment is transmitted using one or multiple Physical Downlink Control Channels (PDCCH). Control information of a PDCCH is transmitted in the control region comprising the first n=1, 2, 3 or 4 OFDM symbols in each subframe where n is the Control Format Indicator (CFI). Typically the control region may comprise many PDCCH carrying control information to multiple user equipments simultaneously. A downlink system with 3 OFDM symbols allocated for control signaling, for example the PDCCH, is illustrated in FIG. 3 and denoted as control region. The resource elements used for control signaling are indicated with wave-formed lines and resource elements used for reference symbols are indicated with diagonal lines. Frequencies f or subcarriers are defined along an z-axis and symbols are defined along an x-axis.
Problems have been identified with the LTE control channel. One of them is its limited capacity and another is the limitation to rely on common reference signals for demodulation, thus, reducing the performance of the radio communications network.